The goal of this proposal is to explore neuron-target cell interaction in the developing mouse cerebellum. Two neurological mutants, lurcher (+/Lc) and staggerer (sg/sg), have previously been analyzed by constructing mutant - wild-type aggregation chimeras. In lurcher 100% of the Purkinje cells die as a direct effect of Lc gene action. Most of the granule cells (90%) and olivary neurons (75%) die as a secondary, epigenetic consequence of the absence of the Purkinje cells. In staggerer the Purkinje cells are small, ectopic, and reduced in number, and these traits are directly caused by staggerer gene action. Virtually 100% of the granule cells die in the mutant, but, as with lurcher, this death is not a direct effect of the gene. The chemiras thus represent an ideal model system in which one can genetically vary the final number of normal Purkinje cells. Consequences of this quantitative variation on the development of the presynaptic neurons (the granule cells and inferior olivary neurons) and the post-synaptic neurons (the deep cerebellar nuclei) can then be examined. Further, by reducing their total number, each remaining wild-type Purkinje cell will be subjected to an increased pre-synaptic "demand" and the effects of this situation on the structure of the Purkinje cell dendritic arbor can also be examined. Lurcher and staggerer chimeras will be produced. The ratio of the two genotypes in these animals will range from mostly mutant to mostly wild-type with many intermediate values. The numbers of Purkinje cells, granule cells, and inferior olivary neurons (where appropriate) will be compared to determine how much plasticity exists in the Purkinje cell to granule cell ratio. Postnatal tritiated thymidine injection followed by autoradiography will determine whether the granule cells which survive in the chimeras are those which made the first synapses or are there other factors involved. Additional chimeras will be analyzed by Golgi impregnations to assess whether there is an upper limit to the size or complexity of the Purkinje cell dendrite, and how the dendrites of the deep cerebellar nuclear neurons respond to decreased input. In sum, I plan to use genetics as a fine neurobiological tool. The results will add to our increasing knowledge of this important area of developmental neurobiology.